A full description of satellite and landmobile communications systems employing multiple beam antenna arrays is described in U.S. patent application Ser. No. 08/179,953, which is incorporated herein in its entirety by reference. Briefly, a satellite-mobile communication system will be described with reference to FIG. 1. FIG. 1 illustrates a plurality of portable stations 12 in communication via a satellite 10 with a hubstation 14. The hubstation is connected, for example via a local exchange, to a public switched telephone network (PSTN) to allow calls to be placed between the portable phones and any telephone subscriber worldwide, as well as between the portable phones. The satellite receives signals from the portable phones at a relatively low microwave frequency, such as 1600 MHz. At such frequencies, the transmitters in battery operated phones can be efficient and their antennas can be small and omnidirectional. The satellite translates the received signals from 1600 MHz to a higher frequency for relaying to the hubstation.
In this system, complex instantaneous waveform samples are transmitted to a satellite for retransmission by different antenna elements. The samples for different antenna elements are preferably time-multiplexed onto the In-phase and Quadrature (I and Q) components of a feeder link for transmission from the ground station to the multi-element relaying antenna, or vice-versa. The real parts of the complex element signals are multiplexed onto the I channel, for example, and imaginary parts are multiplexed onto the Q channel. Any inter-sample interference arising due to a feeder link bandwidth restriction is accounted for in the generation of the samples at the ground station processing unit, or, in the reverse direction, is removed by processing at the ground station processing unit. In both cases, the same mathematical operation known as beamforming is used and the beamforming coefficients are chosen to account for intersymbol interference in transmission.
In the known prior art systems, the satellite processes the received signals and retransmits the signals back toward earth in coarse or wide beams. There are two reasons why these systems which only use coarse beams are not always effective, although the total capacity is the same as with the inventive combination of coarse and free beams. When using coarse beams with a capacity of 500 channels wherein each beam reuses the same frequency spectrum, the signals using the same channel frequency must be approximately one coarse beamwidth apart so as to avoid interfering with each other. The second disadvantage of using only coarse beams is that higher transmitter power is required to communicate when using a coarse beam. This is the principal reason for preferring narrower beams, that is, the achievement of high capacity with less total satellite or terminal transmitter power than with coarser beams. However, when using only narrow beams, there must be a larger number of beams to cover the service region and thus, if total capacity is divided between the beams, the capacity per beam is low. The transmitter power for communications is however reduced. When traffic distributions are not evenly spread but comprise clumps as in major cities, the capacity of a narrow beam may not be enough. Thus, the present invention seeks to provide both the power advantage of narrow beams with the high spot-capacity of large beams to overcome the deficiencies of the prior art.